Stamped brake drums are about 95 percent of brake drums out there. Characterized by a fairly thin section and a "rolled" edge.
Cast drums tend a bit thicker, have an edge that is "square & machined" and can be characterized by a "sandcast" look of the external drum surface (i.e. the casting sand leaves its own imprint in the solidified metal - this on older cast drums.)
Here are pix of original drums and backing plates removed as a unit. Note the rolled edges of both drum and backing plate which nearly "fit together."
Here are cast drums. Note the thicker section and the squared off edge.
Joe K